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ASTRONOMY and ASTROPHYSICS Is the FIP Effect Present Inside Solar Photospheric Magnetic flux Tubes?

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ASTRONOMY and ASTROPHYSICS Is the FIP Effect Present Inside Solar Photospheric Magnetic flux Tubes? Astron. Astrophys. 351, 701–706 (1999) ASTRONOMY AND ASTROPHYSICS Is the FIP effect present inside solar photospheric magnetic flux tubes? V.A. Sheminova1 and S.K. Solanki2;3 1 Main Astronomical Observatory of Ukrainian NAS, Goloseevo, 252650 Kiev-22, Ukraine 2 Institute of Astronomy, ETH-Zentrum, 8092 Zurich,¨ Switzerland 3 Max-Planck-Institute of Aeronomy, 37191 Katlenburg-Lindau, Germany Received 7 May 1999 / Accepted 8 September 1999 Abstract. The first determination of the elemental composition Here we investigate the possibility that the segregation in the photospheric layers of solar magnetic flux tubes is de- has (partly or completely) already taken place in the photo- scribed. Stokes I and V profiles of 13 elements observed in solar spheric layers of magnetic features, so called magnetic ele- active region plage and in the network are analysed. The abun- ments, thought to be composed of magnetic flux tubes (e.g., dances are obtained for elements with high (C, O) and low (Al, Solanki 1993). Such an effect could reproduce the observed Ca, Cr, Na, Ni, Sc, Si, Ti, Y, Zn) first ionization potential (FIP) FIP effect, since in the chromosphere the magnetic elements in order to investigate to what extent the abundance anomalies expand, forming a magnetic canopy (Giovanelli & Jones 1982, observed in the upper solar atmosphere (FIP-effect) are already Solanki & Steiner 1990), and finally fill the whole upper atmo- present in the photospheric layers of flux tubes, which are the sphere, so that the gas exhibiting the FIP effect is connected source of much of the gas in the upper atmosphere. Various in the photosphere almost exclusively to the magnetic elements sources of error are considered and the uncertainties introduced and to sunspots. Note that even if photospheric flux tubes exhibit by them are estimated. There are hints of a weak FIP-effect in a FIP effect of the same magnitude as witnessed in the upper the flux tubes, corresponding to an overabundance of a factor of solar atmosphere, we would still not expect it to be noticeable in 1.1–1.2 of the low-FIP elements relative to high-FIP elements, standard photospheric abundance determinations, because mag- as compared to the quiet photosphere. However, our data set a netic elements cover only 1% or less of the solar surface in firm upper limit of 1.3–1.6 on this factor, which is well below the photospheric layers. enhancement seen in many parts of the upper solar atmosphere. Even if magnetic elements were to exhibit no FIP effect, an analysis of the elemental abundances in magnetic elements Key words: Sun: abundances – line: formation – Sun: photo- still provides a firm lower boundary condition for any model of sphere – Sun: magnetic fields FIP segregation in the chromosphere. In fact, to our knowledge elemental abundances have never been determined in flux tubes before (with the exception of sunspots, which are not studied 1. Introduction here). In the present paper we determine abundances of elements It is well established that in the solar corona, in the slow so- inside magnetic features relative to the iron abundance there. As lar wind and among solar energetic particles the abundances of we shall explain in Sect. 3, it is currently not possible to obtain elements with first ionization potential (FIP) below 10 eV are absolute abundances. However, this is not a serious disadvan- enhanced relative to those with higher first ionization potential tage in the context of the FIP effect, which concerns relative when compared to the abundance ratio measured in the photo- abundance differences. sphere. This variation of relative solar abundances in different atmospheric regions is called the FIP effect (see, e.g., Meyer 1985; von Steiger & Geiss 1989; Reames et al. 1994; Feldman 2. Observational data and selected spectral lines & Laming 1994; Sheeley 1996; Bochsler 1998; Feldman 1998; The observations were made on April 29–30, 1979 with the Geiss 1998, for reviews). McMath-Pierce facility and the 1m Fourier Transform Spec- There is as yet no single generally accepted physical mech- trometer (FTS) polarimeter of the National Solar Observatory anism for the FIP effect. However, the idea underlying most at Kitt Peak. The data consist of Stokes I and V spectra with a theoretical approaches is that the segregation of high and low spectral resolving power of λ/∆λ = 360 000–420 000, a noise FIP elements takes place in the chromosphere or possibly the 4 level of approximately 10− in units of the continuum intensity, transition region (e.g., Henoux 1998). Nevertheless, it is worth- Ic, and a spatial resolution of approximately 1000. A detailed while also to consider alternatives. description of these data is given by Stenflo et al. (1984). Here Send offprint requests to: S.K. Solanki ([email protected]) we analyse four spectrograms, 2 each observed in the network 702 V.A. Sheminova & S.K. Solanki: Is the FIP effect present inside flux tubes? (identified as FTS2 and FTS3) and in an active-region plage the spatially unresolved observations (we call this procedure (FTS4, FTS5). Together FTS2 and FTS3, respectively FTS4 1.5-D radiative transfer). and FTS5 cover a wavelength range from 457.4 to 685.8 nm. A height-independent micro- and macroturbulence of 1 1 The overlapping wavelength ranges between FTS2 and FTS3, 1kms− and2kms− , respectively, were introduced in the flux 1 1 respectively FTS4 and FTS5 were used to give the V profiles in tubes, and of 0.8 km s− and 1.7 km s− in the quiet sun. The both spectra referring to the same kind of solar region a common empirical factor to the Van der Waals damping constant, calcu- polarisation scale, corresponding to the same magnetic filling lated using the formula given by Unsold¨ (1955), is chosen to be factor (see Solanki et al. 1986). The filling factor is the fraction δΓ =2.5. These are the same values as those used to construct of the surface area covered by magnetic field. the empirical flux tube models we use for the line calculations We have selected unblended lines of different elements (Solanki 1986; Solanki & Brigljevic´ 1992). However, we have within this wavelength range using the line list of Gurtovenko also tested the influence of varying these and other parameters & Kostik (1989). After individually checking all the selected (see Sect. 4). The Lande´ factors and splitting patterns of the se- Stokes V profiles, some lines were excluded from the initial lected lines of Al, C, O, Na, Si, Ca, Y, Zn have been determined list due to the presence of small blends, or the weakness of the assuming LS coupling. For Sc, Ti, Cr, Fe, Ni the empirical val- Stokes V signal, which make them susceptible to noise. The ues of gl and gu, taken from the tables of Sugar & Corliss (1985), weak lines (with relative depth d<0.1, where d =1 Il/Ic) were employed instead. Here gl and gu are the Lande´ factors of are most affected by noise. They have generally been dropped,− the lower and upper state of the transition. Wherever available although a few weak lines of O and C have been retained be- with sufficient accuracy, i.e. of the Fe I, Ti I, and Cr I lines, cause these elements have no lines with d>0.1 in the observed the statistically weighted oscillator strengths, gf, obtained by wavelength range. On the other hand, the strong lines (d>0.75, Blackwell’s group (e.g., Blackwell et al. 1982) have been used, W>85 mA)˚ are affected by saturation and NLTE effects, mak- otherwise we employed those of Gurtovenko & Kostik (1989). ing them relatively unreliable abundance indicators. They have We have determined the amplitudes of the blue and red wings, therefore also been dropped. Our final list consists of 93 spectral ab, and ar, of the observed Stokes V profiles and found the av- lines of 13 elements. The elements, their FIP and the number erage amplitudes aV =(ab +ar)/2 for each line in the network of lines analysed for each element are listed in Table 1. Unfor- (FTS2, FTS3) and in the plage (FTS4, FTS5). The central line tunately, only 2 elements, C and O, have a FIP > 10 eV, since depths observed in the quiet sun were taken from Gurtovenko the present data set was not observed with the aim of deriving & Kostik (1989). abundances. Nevertheless, it is unique in the sense that no other In a first step we determined the element abundances for the Stokes V spectrum combining such low noise with such a wide quiet photosphere by fitting the observed central depths of all wavelength range has been obtained before or since. selected lines using the HSRA model. The obtained abundances, AHSRA, as well as their standard deviation are listed in the 4th column of Table 1. The remaining columns list the following: The element and ion, the corresponding first ionization poten- 3. Analysis technique, calculations and results tial, element abundances derived using 1.5-D radiative transfer Our determination of the element abundances is based on the as- in the network and in plages with the NET and PLA models of sumption of LTE and makes use of empirical models of the solar Solanki & Brigljevic´ (1992) and with the plage flux-tube model network (which we shall call NET) and plage flux tubes (PLA) (PLAOLD) of Solanki (1986) (respectively labeled ANET;2, constructed using Fe I, Fe II and C I lines (Solanki & Brigljevic´ APLA;2 and APLAOLD;2) and abundances obtained using 1-D 1992), as well as the standard quiet sun atmosphere (HSRA) radiative transfer with the PLA model of Solanki & Brigljevic´ of Gingerich et al.
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